A collaboration option for the more security conscious is Semaphor, from the same company as the SpiderOak secure cloud storage.
An important consideration with any cloud service is the risk of intrusion into the information you’ve saved on the cloud servers. Cloud providers talk a lot about data security to obscure the hard truth that customer data is saved ‘ in the clear’ on their servers and can be accessed by hackers, the cloud company, their staff (approved or not) or government agencies (with or without court order).
All that applies to chats and messages using collaboration services.
Semaphor has encrypted messaging, team collaboration and file sharing with the twist that all the data on their servers is encrypted. Only authorized users have the ‘keys’ to read the information.
Available for Windows, Mac, Linux plus Apple and Android devices.